The app, Cryptoy, was designed by three industrial placement students at GCHQ (Government Communications Headquarters). It was originally part of a project showcasing encryption at Cheltenham Science Festival, and has now been made available to the public after teachers showed interest in using it.

The app works by enabling users to create their own encoded messages, which can be shared via social media, where others can attempt to decipher them. It teaches users about four encryption techniques: shift, substitution, vigenère and enigma.

Cryptoy aims to teach users about the history of encryption, while teaching basic techniques and drawing comparisons with modern code design. “Even though the examples of cryptography demonstrated by Cryptoy are from an earlier era, the design principles their inventors use are the same as those of modern cryptographic designers,” according to GCHQ.

“As well as explaining the challenges of designing good cryptography to pupils, the app also demonstrates how much fun it can be to break cryptographic design that someone else thought was secure,” it says.

The building of the app is part of the GCHQ outreach programme’s aim to increase the number of pupils studying STEM – Science, Technology, Engineering and Maths – subjects at school. “Cryptoy is mainly directed at Key Stage Four students, so ages 14-16,” says the spokesperson. “Its purpose is to encourage an interest in cryptography and the associated academic disciplines, particularly maths.”

The app is currently only available for download on Android tablets. It can be downloaded for free from the GCHQ website or from Google Play. The agency hopes to release an Apple iOS version for iPads in 2015.

GCHQ is one of three Intelligence and Security Agencies in the UK, working alongside MI5 and the Secret Intelligence Service MI6. The organisation says it is unable to disclose the names of the apprentice students who designed Cryptoy.